Integrated circuit (IC) devices, having an integrated circuit (IC) unit and a lead frame which are sealed within a protective enclosure, find wide use in products including consumer electronics, household appliances, computers, automobiles, telecommunications, robotics and military equipment. The IC unit encompasses integrated circuit chips and hybrid integrated circuit modules which include one or more of the IC chips and other electronic components on a plastic or ceramic support base.
A means to electrically interconnect an IC unit to circuitry external of the IC device takes the form of a lead frame. The lead frame is formed from a highly electrically conductive material, such as copper, copper alloy or an iron-nickel alloy, by stamping or etching a metal blank into a plurality of leads (or lead fingers) defining a central area in which the IC unit is mounted. The lead frame comprises typically a mounting paddle and a plurality of separate lead members extending away from a location adjacent to the paddle. In those instances where the paddle is absent, the leads are formed so that the IC unit is supported by the ends of the leads or the ends of the leads are positioned in an abutting or spaced position with the IC unit or the ends of the leads are overlapping the periphery of the IC unit.
The blanked lead frames are, typically, plated in a conventional manner with a layer of nickel over the surface of the lead frame. Nickel plating was intended to serve as a barrier to diffusion of copper to and formation of reactive copper products, such as copper oxides and sulfides, on the surface of the lead frame. Unfortunately, nickel layer in thickness of less than 400 microinches (10.2 micrometers) contains pores through which migration and diffusion of copper to the surface of the lead frame takes place. However, a layer having thickness of greater than 400 microinches tends to crack when the leads are eventually bent.
An attempt to eliminate or at least reduce effects of diffusion of copper through a less than 400 microinches thick nickel layer was made by depositing a thin layer of palladium or palladium/nickel alloy on top of the nickel layer. (See European Patent Application No. 0 250 146 published Dec. 23, 1987). However, copper corrosion products, including oxides, sulfides and other reaction products of copper, continued to appear on the lead frame, discoloring the surface of the lead frame and degrading its solderability. A further attempt to overcome these shortcomings was made by plating the copper base with a plurality of layers including, in an ascending order from the copper base, a 5 microinch (127 nanometers) thick nickel strike layer, a 3 microinch (76 nanometers) thick palladium/nickel alloy layer, a nickel layer and a palladium layer. The nickel strike layer and the palladium/nickel alloy layer were intended to act as a barrier to copper ion migration to the surface of the lead frame so as to permit the use of a thinner (less than 400 microinches) nickel layer. (See European Patent Application No. 0 335 608 published Oct. 4, 1989). However, this combination of layers also did not lead to a product which could withstand the effects of processing steps required in the process of fabrication of the encapsulated devices. Therefore, there is a need for a coating or a combination of coatings which could provide satisfactory coverage of base metals.